A system for changing the driving speed of a compressor over a wide range has been used heretofore. In this system, an induction motor operatively connected to a compressor is driven by a variable-voltage and variable-frequency (VVVF) inverter. As an inverter of this type, there is known a three-phase inverter circuit having six transistors in a Graetz connection each having a current recirculating diode inversely connected in parallel therewith. With such an inverter, the pulse widths for turning on and off the transistors are controlled so as to obtain a desired output frequency and voltage upon input of a constant d.c. voltage. Various methods of calculating the pulse widths have been proposed, so the details thereof are omitted herein. A d.c. current supplied to the inverter is converted into a variable-voltage and variable-frequency pseudo sine wave a.c. current through the turn-on/off of transistors.
The load torque of a compressor generally changes with a rotary position during one revolution. The load torque change causes rotation vibrations, particularly large vibrations within a low frequency range.
Furthermore, the load torque charge causes a higher or lower actual voltage than an optimum voltage, thereby lowering the motor efficiency.
Still further, pulse widths within a range where both the inverter output frequency and voltage are low, become too narrow, thereby generating large vibration sounds in synchronism with the turn-on/off of inverter switching elements.